Field of the Invention
The present invention relates to a driving circuit for use in a ferroelectric liquid crystal display device using ferroelectric liquid crystals, in particular, to a ferroelectric liquid crystal display device capable of gray scale display of uniform analog tones even when a large display area is involved in a liquid crystal panel.
Description of the Prior Art
Ferroelectric liquid crystals will form a layer structure and have spontaneous polarization orthogonally crossing the molecular long-axes of the liquid crystals. If the spiral structure of the liquid crystal cell layer is unwound by thinning the thickness of the layer, the molecules of the liquid crystals interact with substrates, exhibiting a bistability. Also, their spontaneous polarization interacts with an electric field applied thereto, giving an extremely high response speed. Further, exhibiting a rapid non-linear response, the ferroelectric liquid crystals are suitably available for high contrast display without adding active elements, allowing a high quality display with low cost. Such a high contrast display with ferroelectric liquid crystals is described in detail in the U.S. Pat. No. 4,367,924.
Ferroelectric liquid crystal panels, although of bi-level display in black and white microscopically, involve mixtures of black and white state during a process of switching from black to white, thus being capable in principle of half-tone or gray scale display. In the simple matrix driving without using any active element, a conventional driving waveform for gray scale display is disclosed, for example, in the U.S. Pat. No. 5,061,044. It is described in this document that the prior-art example may be arranged to provide a gray scale display by applying a signal voltage having an intermediate voltage level between ON and OFF levels for gray scale display.
A conventional driving circuit for ferroelectric liquid crystal device is disclosed, for example, in the U.S. Pat. No. 4,709,995. The driving circuit comprises a row drive unit for applying a scanning signal and a column drive unit for generating a signal voltage. The signal voltage is determined in value according to contents of a frame memory for each pixel data.
However, the method as disclosed in the U.S. Pat. No. 5,061,044 has difficulty in providing a gray scale display of uniform tones if a threshold voltage fluctuates because of nonuniformity in thickness of the panel. Since the orientation of the ferroelectric liquid crystals is switched depending on the interaction between the electric field and the spontaneous polarization of the ferroelectric liquid crystals, the threshold voltage tends to vary due to the variation of the thickness of the liquid crystal cell layer.
Further, since the ferroelectric liquid crystal cell layer is small in thickness as thin as 2 .mu.m or so and moreover has spontaneous polarization, electric capacity of the liquid crystal layer is Greater than that of STN, causing a so-called electrode decay to be likely to occur, i.e. a phenomenon that the waveform becomes less sharp around the tips of electrodes. The threshold voltage will vary also depending on temperature.
These reasons account for the tendency that the nonuniformity within the liquid crystal panel will easily occur.
Conventional driving circuits were arranged to produce an intermediate voltage according to video data. However, since the nonuniformity of thickness within the panel that may actually be involved is not compensated, a uniform Gray scale display cannot be achieved. In particular, the matrix driving method is required to have a rapid threshold characteristic, and therefore the number of Gray scale levels or tones to be displayed is limited to small one.
In the U.S. Pat. No. 4,840,462, disclosed is an arrangement of a ferroelectric liquid crystal display in which the amount of charges to be supplied is varied depending on gray scale data with the voltage for driving the active element. Another example as disclosed in the Japanese Patent Laid-Open (Unexamined) Publication SHO 60-66235 is such that the voltage is controlled while the amount of polarized charges is detected, thus attempting to obtain uniform gray scale display.
Both of the U.S. Pat. No. 4,840,462 and the Japanese Patent Laid-Open SHO 60-66235 have suggested methods for solving the above-mentioned problems by controlling the amount of polarized charges of liquid crystals. However, the amount of polarized charges was measured as the sum of the polarization of charges such as electrons and ionic impurities due to the dielectric component and the spontaneous polarization of the ferroelectric liquid crystals. The capacity by the dielectric component is affected by the nonuniformity of the thickness of the cell layer within the liquid crystal panel, while the concentration of ionic impurities would be made nonuniform by injection process of liquid crystals or the like.
For this reason, the conventional methods, which are not arranged to measure only the amount of spontaneous polarization, cannot exclude the effect of the nonuniformity within the panel, so that uniform gray scale display cannot be offered. Also, either of the U.S. Pat. No. 4,840,462 or the Japanese Patent Laid-Open SHO 60-66235 has no description upon some methods for driving a liquid crystal device by a simple matrix driving manner.
Ferroelectric liquid crystals, although exhibiting bistability, are brought into gray scale display state during the process of transition of switching between black and white display conditions. Nonetheless, the ferroelectric liquid crystal device has difficulties in achieving gray scale display of continuous tones, which is mainly ascribed to nonuniformity in threshold characteristics among sites within the liquid crystal panel. Accordingly, whether or not the gray scale display of continuous tones can be realized with ferroelectric liquid crystals depends on how such nonuniformity in threshold characteristics can be corrected.
The U.S. Pat. No. 4,840,462 and the Japanese Patent Laid-Open SHO 60-66235 disclose methods of controlling the amount of polarized charges of a liquid crystal device, whereas the amount of polarized charges is an index which involves the nonuniformity of the liquid crystal panel. Therefore, the conventional methods cannot sufficiently compensate the nonuniformity within the panel. In those methods, pulse voltage is used for driving a liquid crystal panel, where, at the first step, a current for charging the capacity component according to the dielectric constant of liquid crystals flows in a large amount, thereby causing a voltage to be applied to the liquid crystal layer, and then switching of spontaneous polarization of liquid crystal molecules will start with some delay.
Charge Q1 due to the dielectric component can be derived according to the following equation: EQU Q1=CV=.epsilon..sub.LC .epsilon..sub.O SV/d, (1)
where if the relative dielectric constant of liquid crystals .epsilon..sub.LC =7, area S=1 cm.sup.2, thickness d=1.8 .mu.m, .epsilon..sub.O =8.854.times.10.sup.-12 F/m, voltage V=25 volts, then the resulting Q1=86 nC.
On the other hand, if molecules of the ferroelectric liquid crystals are switched in direction (i.e., display is switched between black and white), the spontaneous polarization is switched, causing a Ps switching current to flow. Once a pixel that has been reset into a black state with a one-polarity pulse has an opposite-polarity pulse applied thereto, the molecules will start being switched into a white state. At this time, the spontaneous polarization is switched from -Ps to +Ps, so that the amount of accumulated charges that flow will be the product of the doubled Ps and the switched area.
Charge Q2 due to the ferroelectric component is represented by the product of the amount of spontaneous polarization Ps per unit area of liquid crystals and the pixel area: EQU Q2=2Ps.multidot.S (2)
where if the spontaneous polarization Ps is assumed to be 20 nC/cm.sup.2, Q2 will be 40 nC. Assuming here that the thickness d has a nonuniformity of .+-.0.1 .mu.m in equation Q1, the error of Q1 becomes approximately 9.5 nC, which is generally equal to 1/4 of Q2. Q2 reflects the density of the pixel accurately, however, even if the polarized charge of the sum of Q2 and Q1 is controlled, only a slight nonuniformity in thickness as small as .+-.0.1 .mu.m will make it difficult to provide a uniform gray scale display of continuous tones.
Further, the threshold characteristic by which the spontaneous polarization is switched will be varied depending on the orientation of liquid crystal molecules and ionic impurities, which leads to variance in response characteristic of molecules (spontaneous polarization) due to any nonuniformity in a rubbing orientation treatment or injection process of liquid crystals.
In consequence, the conventional methods cannot be free from any influence of the nonuniformity within the panel.